Base64

The command base64 is one of several data encodings provided by the package trf. See trf-intro for an overview of the whole package.

This encoding transforms every block of three bytes into a block of four bytes, each of which is printable, i.e. 7bit ASCII. This implies that the result is valid UTF-8 too. The command uses essentially the same algorithm as for uuencode, except for a different mapping from 6-bit fragments to printable bytes.

base64 options… data

-mode encode|decode

This option has to be present and is always understood by the encoding.

For immediate mode the argument value specifies the oper- ation to use. For an attached encoding it specifies the operation to use for writing. Reading will automatically use the reverse operation. See section IMMEDIATE versus ATTACHED for explanations of these two terms.

Encode converts from arbitrary (most likely binary) data into the described representation, decode does the reverse .

-attach channel

The presence/absence of this option determines the main operation mode of the transformation.

If present the transformation will be stacked onto the channel whose handle was given to the option and run in attached mode. More about this in section IMMEDIATE ver- sus ATTACHED.

If the option is absent the transformation is used in immediate mode and the options listed below are recog- nized. More about this in section IMMEDIATE versus ATTACHED.

-in channel

This options is legal if and only if the transformation is used in immediate mode. It provides the handle of the channel the data to transform has to be read from.

If the transformation is in immediate mode and this option is absent the data to transform is expected as the last argument to the transformation.

-out channel

This options is legal if and only if the transformation is used in immediate mode. It provides the handle of the channel the generated transformation result is written to.

If the transformation is in immediate mode and this option is absent the generated data is returned as the result of the command itself.

NOTES

[1] The encoding is equivalent to PGP’s ASCII armor and was also accepted as one of the MIME encodings for encapsulation of binary data. See RFC 2045 tor.org/rfc/rfc2045.txt) for details and the specification of this encoding.

[2] The encoding buffers 2 bytes.

IMMEDIATE versus ATTACHED

The transformation distinguishes between two main ways of using it. These are the immediate and attached operation modes.

For the attached mode the option -attach is used to associate the transformation with an existing channel. During the execution of the command no transformation is performed, instead the channel is changed in such a way, that from then on all data written to or read from it passes through the transformation and is modified by it according to the definition above. This attachment can be revoked by executing the command unstack for the chosen channel. This is the only way to do this at the Tcl level.

In the second mode, which can be detected by the absence of option -attach, the transformation immediately takes data from either its com- mandline or a channel, transforms it, and returns the result either as result of the command, or writes it into a channel. The mode is named after the immediate nature of its execution.

Where the data is taken from, and delivered to, is governed by the presence and absence of the options -in and -out. It should be noted that this ability to immediately read from and/or write to a channel is an historic artifact which was introduced at the beginning of Trf’s life when Tcl version 7.6 was current as this and earlier versions have trouble to deal with \0 characters embedded into either input or out- put.